Catch more unquantifiable elements (#391)

This adds start/end anchors ^ and $, groups that form zero-width
assertions, and option-changing groups without content `(?i)...`

Author: natecook1000

Sources/_RegexParser/Regex/AST/AST.swift

@@ -125,7 +125,9 @@ extension AST.Node {
     switch self {
     case .atom(let a):
       return a.isQuantifiable
-    case .group, .conditional, .customCharacterClass, .absentFunction:
+    case .group(let g):
+      return g.isQuantifiable
+    case .conditional, .customCharacterClass, .absentFunction:
       return true
     case .alternation, .concatenation, .quantification, .quote, .trivia,
         .empty:

Sources/_RegexParser/Regex/AST/Atom.swift

@@ -808,6 +808,8 @@ extension AST.Atom {
     // TODO: Are callouts quantifiable?
     case .escaped(let esc):
       return esc.isQuantifiable
+    case .startOfLine, .endOfLine:
+      return false
     default:
       return true
     }

Sources/_RegexParser/Regex/AST/Group.swift

@@ -136,3 +136,18 @@ extension AST.Group {
     }
   }
 }
+
+extension AST.Group {
+  var isQuantifiable: Bool {
+    switch kind.value {
+    case .capture, .namedCapture, .balancedCapture, .nonCapture,
+        .nonCaptureReset, .atomicNonCapturing, .scriptRun, .atomicScriptRun,
+        .changeMatchingOptions:
+      return true
+      
+    case .lookahead, .negativeLookahead, .nonAtomicLookahead,
+        .lookbehind, .negativeLookbehind, .nonAtomicLookbehind:
+      return false
+    }
+  }
+}

Tests/RegexTests/ParseTests.swift

@@ -2652,6 +2652,10 @@ extension RegexTests {
     diagnosticTest(#"\Z??"#, .notQuantifiable)
     diagnosticTest(#"\G*?"#, .notQuantifiable)
     diagnosticTest(#"\z+?"#, .notQuantifiable)
+    diagnosticTest(#"^*"#, .notQuantifiable)
+    diagnosticTest(#"$?"#, .notQuantifiable)
+    diagnosticTest(#"(?=a)+"#, .notQuantifiable)
+    diagnosticTest(#"(?i)*"#, .notQuantifiable)
     diagnosticTest(#"\K{1}"#, .unsupported(#"'\K'"#))
     diagnosticTest(#"\y{2,5}"#, .notQuantifiable)
     diagnosticTest(#"\Y{3,}"#, .notQuantifiable)